1. Field of the Invention
The present invention relates to a battery pack and a remaining battery power calculation method, and more particularly, to a battery pack and a remaining battery power calculation method which permits calculating remaining battery power according to the number of charge/discharge cycles and temperature during use of the battery.
2. Related Art
Battery packs (secondary batteries) such as liquid ion batteries have specific capacities, and their capacities have the characteristic of varying according to the temperature under which the battery pack is used.
When a battery pack is used under low temperature, the internal impedance of its battery cell increases and when the same current as normal temperature is applied, a large voltage drop occurs, so the capacity of the battery pack decreases.
FIG. 11 is a graph showing discharge characteristics of a battery pack at 25° C., 10° C. and 0° C. The abscissa represents time, while the ordinate represents voltage.
As shown in FIG. 11, in the case where its discharge power and end voltage are set to 2.0 W and 3.35 V, respectively, the following measurement result is obtained: if a dischargeable capacity in an environment of, for example, 25° C. is set to 100%, approximately 80% dischargeable capacity and approximately 60% dischargeable capacity are respectively obtained in environments of 10° C. and 0° C.
On the other hand, the battery pack has the characteristic of decreasing its capacity even in the case where it continues to be used and the number of charge/discharge cycles increases. This is because the battery pack repeats a charge/discharge cycle and its battery cell is degraded, so the usable capacity thereof decreases, and called ‘cycle degradation’.
The usable capacity of a battery which has undergone a large number of charge/discharge cycles is changed by two factors: cycle degradation and temperature during use.
The technique of detecting the temperature of a battery pack by using a temperature sensor and correcting the remaining battery power has heretofore been used to correct battery capacity diminutions during use at low temperatures, as in Japanese Laid-Open Patent Application JP-A-2000-260488 (Paragraph Numbers [0038]-[0072]), for example.
Similarly, the technique of counting the number of charge/discharge cycles, determining that cycle degradation is advancing, according to the counted number, and estimating the remaining battery power to be lower than the actual battery power is used to correct battery degradation due to the number of charge/discharge cycles.
In these conventional techniques, correction of a usable time in the current temperature environment is performed by measuring the current temperature with a thermistor, and correction of the usable time of a battery cell which has undergone a larger number of charge/discharge cycles is performed by counting the number of charge/discharge cycles.
Specifically, temperature data and correction values depending on temperate are held, and when temperature data indicates a low temperature, the remaining battery power is estimated to be lower than the actual remaining battery power, in view of the degradation of the discharge characteristic of the battery cell. Similarly, correction values for cycle degradation are prepared, and as the number of charge/discharge cycles becomes larger, the remaining battery power is estimated to be smaller. In this manner, two correction values, i.e., temperature and cycle degradation, are prepared as data stored in the battery pack, whereby the remaining battery power is corrected according to use environment and the number of times of use.